Consideration will be given of downlink transmission from a base station to a terminal in Multiple Input Multiple Output (MIMO) communication in which multiple antennas are disposed on both sending and receiving sides. In MIMO communication in closed-loop systems, a base station carries out beamforming based on channel information fed back from a terminal, thereby improving communication quality and system capacity. For example, in Frequency Division Duplexing (FDD) systems, since uplink and downlink channel responses differ from each other and are unknown, channel information estimated by a terminal is fed back to a base station when transmission beamforming is carried out.
However, direct feedback of channel information causes heavy loads on a network. For example, in a MIMO system where a transmitter has N antennas and a receiver has M antennas, channels between the transmitter and the receiver are the values of N×M complex numbers, resulting in the increased amount of feedback information. Therefore, a feedback method using a codebook is employed to reduce the amount of feedback information. According to this method, a table of precoding matrixes (a codebook) is shared beforehand between a terminal and a base station, and the terminal feeds back the base station with, as channel information, an index to a most relevant precoding matrix based on an estimated downlink channel response. Such an index in the codebook is referred to as Precoding Matrix Indicator (PMI). The base station, based on a fed back PMI, determines a precoding matrix from the codebook and multiplies a transmission signal by it, whereby it is possible to control beamforming for each terminal.
For beamforming in horizontal direction, it is possible to perform codebook-based beamforming over an entire cell coverage by using, for example, a precoding matrix as described in NPL 1.
For beamforming in vertical direction, it is possible to calculate a precoding matrix that realizes a desired beam angle in vertical direction, as described in NPL 2. Here, a beam angle in vertical direction is defined as an angle at which a base station looks down in the direction of the main beam of a beam realized by a precoding matrix.